Redundant hybrid ring laser

ABSTRACT

The disclosed embodiments improve on the design of existing hybrid ring lasers by enabling a redundancy of one of the least reliable components, the III-V reflective semiconductor optical amplifier (RSOA). This allows a spare RSOA to be used to replace a failed RSOA while using the same ring mirror as the wavelength selective filter, thus reducing link down time, and eliminating the need for additional switching or multiplexing elements which add excess loss and require additional power. The result is a more reliable transmitter enabling greater scale in networking systems. In addition, this facilitates a widely tunable laser with the same outputs by utilizing two gain media comprised of different bandgap active material. Finally, multiple correlated wavelengths can be emitted from this device with two different gain materials using the same ring mirror element as reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with U.S. government support under Agreement No.HR0011-08-9-0001 awarded by DARPA. The U.S. government has certainrights in the invention.

BACKGROUND Field

The disclosed embodiments generally relate to designs forsemiconductor-based lasers. More specifically, the disclosed embodimentsrelate to the design of a redundant hybrid ring laser that providesredundancy for reflective semiconductor optical amplifier (RSOA)components of the laser.

Related Art

Silicon photonics is a promising new technology that can provide largecommunication bandwidth, low latency and low power consumption forinter-chip and intra-chip connections or optical links. In order toachieve low-latency, high-bandwidth optical connectivity, a number ofoptical components are required, including: optical modulators, opticaldetectors, wavelength multiplexers/demultiplexers and optical sourcessuch as lasers.

Some recently developed optical networking systems provide large numbers(e.g., hundreds or thousands) of parallel links, which makes thereliability of every component in each of the links vital to reliabilityof the systems as a whole. Perhaps the single least-reliable componentsin these systems are the group III-V compound semiconductor devices thatare used in fabricating lasers. These devices can be tested thoroughlyduring the manufacturing process, but long-term reliability is notguaranteed. Note that the hybrid ring laser concept provides a scalablesolution to implementing highly parallel transceivers by simplifying thegain medium through use of uniform RSOA components, and replacingpotential failure mechanisms of wavelength-selective mirrors known asdistributed Bragg reflectors (DBRs) with silicon ring filter mirrors.However, providing additional redundancy for the RSOA components couldimprove the reliability of such systems considerably.

Hence, what is needed is a technique for implementing a hybrid ringlaser that facilitates providing redundancy for RSOA components in suchlasers.

SUMMARY

One embodiment of the present disclosure implements an integrated laser.This integrated laser includes a first reflective silicon opticalamplifier (RSOA), a second RSOA, and an optical ring resonator. It alsoincludes a symmetric power splitter comprising a reciprocal networkhaving a first port, a second port, a third port and a fourth port,wherein light entering the first port is approximately equally splitwith a 90-degree phase difference between the third port and the fourthport, and wherein light entering the second port is approximatelyequally split with a 90-degree phase difference between the third portand the fourth port. The integrated laser also includes a number ofwaveguides, including: a first optical waveguide coupled between thefirst port and the first RSOA; a second optical waveguide coupledbetween the second port and the second RSOA; a third optical waveguidecoupled to the third port, which channels light emanating from the thirdport in proximity to the optical ring resonator to cause opticallycoupled light to circulate in a first direction in the optical ringresonator; and a fourth optical waveguide coupled to the fourth port,which channels light emanating from the fourth port in proximity to theoptical ring resonator to cause optically coupled light to circulate ina second direction, which is opposite to the first direction, in theoptical ring resonator. The integrated laser additionally includes anoutput optical waveguide, which is optically coupled to the optical ringresonator.

In some embodiments, light emanating from the first RSOA is split by thesymmetric power splitter between the third and fourth opticalwaveguides, which causes optically coupled light to circulate inopposite directions in the optical ring resonator, thereby causingoptically coupled light to reflect back through the third and fourthoptical waveguides to the symmetric power splitter, wherein thereflected light is combined and directed back to the first RSOA, andwherein no reflected light is directed back to the second RSOA due tophase cancellation. Note that the first RSOA, the first, third andfourth optical waveguides, the symmetric power splitter and the opticalring resonator collectively form a first lasing cavity for theintegrated laser.

Similarly, light emanating from the second RSOA is split by thesymmetric power splitter between the third and fourth opticalwaveguides, which causes optically coupled light to circulate inopposite directions in the optical ring resonator, thereby causingoptically coupled light to reflect back through the third and fourthoptical waveguides to the symmetric power splitter, wherein thereflected light is combined and directed back to the second RSOA, andwherein no reflected light is directed back to the first RSOA due tophase cancellation. In this case, the second RSOA, the second, third andfourth optical waveguides, the symmetric power splitter and the opticalring resonator collectively form a second lasing cavity for theintegrated laser.

In some embodiments, only one of the first RSOA and the second RSOA is acurrently active RSOA that is powered on, and the other RSOA ismaintained as a spare RSOA that is not powered on. In these embodiments,the integrated laser also includes a failure-detection mechanism thatdetects a failure or degradation of the first and/or the second RSOA,and a switching mechanism that powers off the currently active RSOA andpowers on the spare RSOA when the failure-detection mechanism detectsthat the currently active RSOA has failed.

In some embodiments, the gain bandwidth of the first RSOA and the gainbandwidth of the second RSOA have different overlapping frequencyranges. Moreover, only one of the first RSOA and the second RSOA ispowered on at a given time depending on a desired wavelength tofacilitate tuning the integrated laser over a combined frequency rangethat includes the overlapping frequency ranges of the first and secondRSOAs.

In some embodiments, the gain bandwidth of the first RSOA and the gainbandwidth of the second RSOA have different distinct frequency ranges,and the first RSOA and the second RSOA are both powered on at the sametime, whereby the integrated laser simultaneously outputs laseremissions in two different wavelength bands.

In some embodiments, the first RSOA and the second RSOA are located onone or more RSOA semiconductor chips, wherein the RSOA semiconductorchips are separate from a semiconductor chip that includes the symmetricpower splitter, the first, second, third and fourth optical waveguides,the optical ring resonator and the output waveguide.

In some embodiments, the optical ring resonator comprises two opticalring resonators, which are optically coupled in a Vernier configurationthat facilitates tuning the optical ring resonator over a wider spectralrange than a single optical ring resonator can be tuned over.

In some embodiments, the output optical waveguide has two ends thatfunction as two outputs of the integrated laser.

In some embodiments, the two ends of the output optical waveguide feedinto two arms of a Mach-Zehnder modulator (MZM), which modulates signalsreceived from the output optical waveguide.

In some embodiments, the integrated laser includes one or moreadditional output optical waveguides, which are optically coupled to theoptical ring resonator, and wherein each additional output waveguide hastwo ends that function as outputs of the integrated laser.

In some embodiments, a modulator is coupled to each output of theintegrated laser, wherein each output provides an optical carrier signalthat is modulated by the associated modulator to communicateinformation.

In some embodiments, the symmetric power splitter comprises adirectional coupler, or a multi-mode interferometer.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1A illustrates an integrated laser in accordance with the disclosedembodiments.

FIG. 1B illustrates a first lasing cavity for the integrated laser inaccordance with the disclosed embodiments.

FIG. 1C illustrates a second lasing cavity for the integrated laser inaccordance with the disclosed embodiments.

FIG. 2 illustrates how the integrated laser can be operated in adual-band mode in an implementation that provides multiple outputs inaccordance with the disclosed embodiments.

FIG. 3 illustrates how the integrated laser can be operated in adual-band mode in an implementation that includes two separate RSOAchips in accordance with the disclosed embodiments.

FIG. 4A illustrates how a general symmetric mirror can be used in placeof an optical ring resonator in accordance with the disclosedembodiments.

FIG. 4B illustrates how two optical ring resonators in a Vernierconfiguration can be used to implement a symmetric mirror in accordancewith the disclosed embodiments.

FIG. 4C illustrates how two identical mirrors can be used to implement asymmetric mirror in accordance with the disclosed embodiments.

FIG. 4D illustrates another implementation for a symmetric mirror inaccordance with the disclosed embodiments.

FIG. 5 illustrates gain spectra and laser spectra for a dual-bandimplementation of the integrated laser in accordance with the disclosedembodiments.

FIG. 6 presents a flow chart illustrating how a hybrid laser operates inaccordance with the disclosed embodiments.

FIG. 7 presents a flow chart illustrating how a failed RSOA can bereplaced in accordance with an embodiment of the present disclosure.

FIG. 8 illustrates a system that includes an optical source, such as anintegrated laser, in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the present embodiments, and is provided in thecontext of a particular application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the present embodiments. Thus, the presentembodiments are not limited to the embodiments shown, but are to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing computer-readable media now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium. Furthermore, the methodsand processes described below can be included in hardware modules. Forexample, the hardware modules can include, but are not limited to,application-specific integrated circuit (ASIC) chips, field-programmablegate arrays (FPGAs), and other programmable-logic devices now known orlater developed. When the hardware modules are activated, the hardwaremodules perform the methods and processes included within the hardwaremodules.

Various modifications to the disclosed embodiments will be readilyapparent to those skilled in the art, and the general principles definedherein may be applied to other embodiments and applications withoutdeparting from the spirit and scope of the present invention. Thus, thepresent invention is not limited to the embodiments shown, but is to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

Redundant Hybrid Laser

The disclosed embodiments improve the design of existing hybrid ringlasers by facilitating redundancy of one of the least reliablecomponents, the III-V reflective semiconductor optical amplifier (RSOA).This enables a spare RSOA to be used in place of a failed RSOA whileusing the same ring mirror as the wavelength selective filter, therebyreducing link down time, and eliminating the need for additionalswitching or multiplexing structures, which can add excess loss andconsume additional power. The result is a more reliable transmitter thatenables greater scalability in networking systems. This device can alsobe used to implement a widely tunable laser by utilizing two gain mediacomprised of different bandgap active material. Also, multiplewavelengths can be emitted from this device by using different gainmaterials with the same ring mirror element.

Unlike existing hybrid ring lasers, the disclosed design uses two RSOAsinstead of a single RSOA, and also uses a symmetric power splitter (alsoreferred to as a “3 dB coupler”) to couple the RSOAs to the ring-basedmirror in a way that provides an appropriate phase relationship tofacilitate replacing a failed RSOA without additional switchingstructures. The phase relationship of the symmetric power splitter issuch that with only the first RSOA powered on, power will not bereflected back from the ring mirror into the second RSOA, thusincreasing the power loss. Instead, all of the power reflected by thering mirror will be coupled back to the first RSOA. Hence, this systemactually comprises two separate lasers, which share a single ring forwavelength selection. This system is described in more detail below.

This new hybrid laser provides significant advantages in comparison toexisting hybrid lasers because it facilitates replacing the leastreliable component, the III-V gain medium, with a spare III-V gainmedium resulting in a significantly more reliable laser, without: (1)the need for additional switching structures; (2) the need to providepower-splitting for the laser; or (3) the need for manual maintenanceoperations to replace the III-V gain medium. This solution maintains allof the advantages of existing hybrid ring lasers (e.g., efficiency, highSMSR, tunability, narrow linewidth, silicon photonic integration) withincreased reliability.

In a widely tunable configuration, this system has the potential toprovide the world's most widely tunable semiconductor laser. Moreover,with an athermalized circuit, this system has the potential to be a WDMlaser with a fixed grid, and thereby meets a critical specification forWDM links.

More specifically, FIG. 1A illustrates an exemplary redundant hybridlaser 100, which can be configured to have a single output or sharedoutputs in accordance with the disclosed embodiments. Hybrid laser 100includes two separate optical gain chips, including III-V gain chip 102and III-V gain chip 104, wherein III-V gain chip 102 includes areflective semiconductor optical amplifier (RSOA) 103 and III-V gainchip 104 includes an RSOA 105.

When RSOA 103 is powered on, it generates an optical signal, which hasan associated carrier or fundamental wavelength λ (such as 1.3 or 1.55μm). This optical signal feeds through a III-V/Si coupler 115 into anoptical waveguide 116 located in a separate photonic chip 101. Opticalwaveguide 116 feeds through a phase modulator 106 into an input of asymmetric power splitter 109 (also referred to as a “3 dB coupler”).Similarly, when RSOA 105 is powered on, it generates an optical signalthat feeds through a III-V/Si coupler into an optical waveguide 117 inphotonic chip 101. Optical waveguide 117 similarly feeds through a phasemodulator 107 into another input of a symmetric power splitter 109.

Symmetric power splitter 109 comprises a reciprocal network having afirst port coupled to optical waveguide 116, a second port coupled tooptical waveguide 117, a third port coupled to optical waveguide 118 anda fourth port coupled to optical waveguide 119. Light entering the firstport is approximately equally split with a 90-degree phase differencebetween the third port and the fourth port, and light entering thesecond port is approximately equally split with a 90-degree phasedifference between the third port and the fourth port.

Optical waveguide 118 channels light emanating from the third port ofsymmetric power splitter 109 in proximity to an optical ring resonator108 to cause optically coupled light to circulate in a first directionin optical ring resonator 108. Similarly, optical waveguide 119 channelslight emanating from the fourth port of symmetric power splitter 109 inproximity to optical ring resonator 108 to cause optically coupled lightto circulate in a second direction, which is opposite to the firstdirection, in optical ring resonator 108.

The light circulating in optical ring resonator 108 causes opticallycoupled light to reflect back through optical waveguides 118 and 119 tothe symmetric power splitter 109, wherein the reflected light iscombined and directed back to the RSOA that originally sent the light,but not to the other RSOA that did not send the light. Note the phaserelationships maintained within symmetric power splitter 109 ensure thatthe reflected light which exits ring resonator 108 is only directed backto the RSOA that originated the light. At the same time, phasecancellation within symmetric power splitter 109 ensures that noreflected light is directed back to the other RSOA.

Optical waveguide 118 is also coupled to a monitor 110, which can beused to determine the alignment between the laser cavity modes with amirror resonance for ring resonator 108. Similarly, optical waveguide119 is coupled to a monitor 114, which can also be used to determine thealignment between the laser cavity modes with a mirror resonance forring resonator 108.

As illustrated in FIG. 1A, hybrid laser 100 can include either a singleMach-Zehnder modulator (MZM) output 112 or 2N outputs 111. In theembodiment that includes a single MZM output 112, two ends of an outputoptical waveguide feed into two arms of an MZM, which modulates signalsreceived from the output optical waveguide.

In the embodiment that includes 2N outputs 111, the hybrid laserincludes one or more output optical waveguides, which are opticallycoupled to the optical ring resonator, and which each have two ends thatfunction as outputs of the integrated laser. If sufficient care is takenin designing these output optical waveguides, the output power will bethe same across all of the outputs of all waveguides. Hence, a systemthat has N such optical waveguides can provide 2N balanced outputs. Notethat there exists a minimum bend radius for these output waveguides thatconstrains the number of output waveguides that can be incorporated intothe system before bending losses arise.

In the design illustrated in FIG. 1B, RSOA 103 and RSOA 105 are locatedin the same III-V gain chip. This differs from the design illustrated inFIG. 1A in which RSOA 103 and RSOA 105 are located in separate III-Vgain chips 102 and 104. Note that locating both RSOAs 103 and 105 in thesame chip can make the system easier to manufacture. However, locatingRSOA 103 and RSOA 105 on separate chips can be advantageous if thewavelengths of the gain media are very different and the fabrication ofquality gain material on the same wafer is a challenge, or is notpossible. In the embodiment illustrated in FIG. 1B, the two output portsof ring resonator 108 are directed into the two arms of MZM 112.

During operation of the system illustrated in FIG. 1B, RSOA 103 ispowered on while RSOA 105 is powered off. The light emanating from RSOA103 flows through optical waveguide 116 and is split by symmetric powersplitter 109 between optical waveguides 118 and 119. This causesoptically coupled light to circulate in opposite directions in opticalring resonator 108, and thereby causes optically coupled light toreflect back through optical waveguides 118 and 119 to the symmetricpower splitter 109, wherein the reflected light is combined and directedback to the RSOA 103, and wherein no reflected light is directed back toRSOA 105 due to phase cancellation. Note that RSOA 103, the opticalwaveguides 116, 118 and 119, symmetric power splitter 109 and opticalring resonator 108 collectively form a first lasing cavity for theintegrated laser as is illustrated in green in FIG. 1B.

FIG. 1C illustrates the same system as appears in FIG. 1B, except thatRSOA 105 is powered on while RSOA 103 is powered off. In this case,light emanating from RSOA 105 flows through optical waveguide 117 and issplit by the symmetric power splitter 109 between optical waveguides 118and 119. This causes optically coupled light to circulate in oppositedirections in optical ring resonator 108, and thereby causes opticallycoupled light to reflect back through optical waveguides 118 and 119 tothe symmetric power splitter 109, wherein the reflected light iscombined and directed back to the RSOA 105, and wherein no reflectedlight is directed back to RSOA 103 due to phase cancellation. In theexample illustrated in FIG. 1C, RSOA 105, the optical waveguides 117,118 and 119, symmetric power splitter 109 and the optical ring resonator108 collectively form a second lasing cavity for the integrated laser asis illustrated in green.

Dual-Band Hybrid Laser

FIGS. 2 and 3 illustrate dual-band variations of the hybrid laser. Morespecifically, FIG. 2 illustrates an implementation with a single sharedIII-V gain chip 102 and 2N outputs 111. In contrast, FIG. 3 illustratesan alternative implementation with separate III-V gain chips 102 and 104and also a symmetric power splitter (3 dB coupler) 115, which isanalogous to symmetric power splitter (3 dB coupler) 109 described abovewith reference to FIG. 1A. Symmetric power splitter (3 dB coupler) 115produces two outputs and operates to divide the light into differentcolors.

In these dual-band variations, rather than keeping only a single RSOApowered on, both can be kept powered on without interference if the gainbandwidth of the two RSOAs does not overlap. For example, FIG. 5illustrates gain spectra 504 and laser spectra 506 for a dual-bandimplementation of the hybrid laser in accordance with the disclosedembodiments. As illustrated in FIG. 5, the combination of thewavelength-selective nature of the gain media and that of the ringresonator resonances 502 combine to provide a WDM output in each of the2N outputs 111 illustrated in FIG. 2.

Hybrid Laser With Increased Tuning Range

This technique could also be used to increase the total tuning range ofthe laser. For example, in some embodiments, the gain bandwidths of twoRSOAs have different overlapping frequency ranges, and only one of thetwo RSOAs is powered on at a given time depending on a desiredwavelength for the integrated laser. This facilitates tuning theintegrated laser over a combined frequency range that includes theoverlapping frequency ranges of the two RSOAs.

Alternative Implementations

In general, the optical ring resonator 108 illustrated in FIGS. 1A, 1B,1C, 2 and 3 can be replaced with any symmetric mirror that returns equalpower and phase back to the third and forth ports of symmetric powersplitter 109. This is illustrated in FIG. 4A, which displays using asymmetric mirror 400 in place of optical ring resonator 108. In someembodiments, this symmetric mirror can be implemented using two opticalring resonators, which are optically coupled together in a Vernierconfiguration, which facilitates tuning the optical ring resonator overa wider spectral range than a single optical ring resonator can be tunedover. For example, see FIG. 4B, which illustrates how two ringresonators 402 and 404 can be arranged in a Vernier configuration toimplement such a symmetric mirror. In other embodiments, the symmetricmirror 400 can be implemented using two identical mirrors 406 and 408 asillustrated in FIG. 4C, or alternatively using a simple loop asillustrated in FIG. 4D.

Operation of the Integrated Laser

FIG. 6 presents a flow chart illustrating how a hybrid laser operates inaccordance with the disclosed embodiments. During operation, the systemgenerates an optical signal by powering up a first RSOA while leaving asecond RSOA unpowered (step 602). Next, the system couples the generatedoptical signal through a first optical waveguide to a first port of asymmetric power splitter, wherein this symmetric power splittercomprises a reciprocal network having a first port, a second port, athird port and a fourth port, wherein light entering the first port fromthe first optical waveguide is approximately equally split with a90-degree phase difference between the third port and the fourth port,and wherein light entering the second port from a second opticalwaveguide, which is coupled to the second RSOA is approximately equallysplit with a 90-degree phase difference between the third port and thefourth port (step 604).

Next, the system channels light emanating from the third port through athird optical waveguide that passes in proximity to an optical ringresonator to cause optically coupled light to circulate in a firstdirection in the optical ring resonator (step 606). The system alsochannels light emanating from the fourth port through a fourth opticalwaveguide that passes in proximity to the optical ring resonator tocause optically coupled light to circulate in a second direction, whichis opposite to the first direction, in the optical ring resonator (step608).

The system then channels light that is reflected back into the opticalring resonator into the third and fourth optical waveguides and thenthrough the symmetric power splitter, wherein the reflected light iscombined and directed back to the first RSOA, wherein no reflected lightis directed back to the second RSOA due to phase cancellation, andwherein the first RSOA, the first, third and fourth optical waveguides,the symmetric power splitter and the optical ring resonator collectivelyform a first lasing cavity for the integrated laser (step 610). Finally,the system provides an output through an output optical waveguide, whichis coupled to the optical ring resonator (step 612).

FIG. 7 presents a flow chart illustrating how a failed RSOA can beswapped in accordance with an embodiment of the present disclosure. Atthe start of this process, the system operates the integrated laser bypowering up the first RSOA while leaving the second RSOA unpowered (step702). Next, upon detecting a failure or degradation of the first RSOA,the system powers off the first RSOA and powers on the second RSOA (step704). In some embodiments, the signaling is only inverted after theswitch of RSOAs is made, thereby enabling the switching to occur betweenpackets in most protocols.

System

One or more of the preceding embodiments of the integrated laser may beincluded in a system or device. More specifically, FIG. 8 illustrates asystem 800 that includes an optical source 802 implemented using anintegrated laser. System 800 also includes a processing subsystem 806(with one or more processors) and a memory subsystem 808 (with memory).

In general, components within optical source 802 and system 800 may beimplemented using a combination of hardware and/or software. Thus,system 800 may include one or more program modules or sets ofinstructions stored in a memory subsystem 808 (such as DRAM or anothertype of volatile or non-volatile computer-readable memory), which,during operation, may be executed by processing subsystem 806.Furthermore, instructions in the various modules in memory subsystem 808may be implemented in: a high-level procedural language, anobject-oriented programming language, and/or in an assembly or machinelanguage. Note that the programming language may be compiled orinterpreted, e.g., configurable or configured, to be executed by theprocessing subsystem.

Components in system 800 may be coupled by signal lines, links or buses,for example bus 804. These connections may include electrical, optical,or electro-optical communication of signals and/or data. Furthermore, inthe preceding embodiments, some components are shown directly connectedto one another, while others are shown connected via intermediatecomponents. In each instance, the method of interconnection, or“coupling,” establishes some desired communication between two or morecircuit nodes, or terminals. Such coupling may often be accomplishedusing a number of photonic or circuit configurations, as will beunderstood by those of skill in the art; for example, photonic coupling,AC coupling and/or DC coupling may be used.

In some embodiments, functionality in these circuits, components anddevices may be implemented in one or more: application-specificintegrated circuits (ASICs), field-programmable gate arrays (FPGAs),and/or one or more digital signal processors (DSPs). Furthermore,functionality in the preceding embodiments may be implemented more inhardware and less in software, or less in hardware and more in software,as is known in the art. In general, system 800 may be at one location ormay be distributed over multiple, geographically dispersed locations.

System 800 may include: a switch, a hub, a bridge, a router, acommunication system (such as a wavelength-division-multiplexingcommunication system), a storage area network, a data center, a network(such as a local area network), and/or a computer system (such as amultiple-core processor computer system). Furthermore, the computersystem may include, but is not limited to: a server (such as amulti-socket, multi-rack server), a laptop computer, a communicationdevice or system, a personal computer, a work station, a mainframecomputer, a blade, an enterprise computer, a data center, a tabletcomputer, a supercomputer, a network-attached-storage (NAS) system, astorage-area-network (SAN) system, a media player (such as an MP3player), an appliance, a subnotebook/netbook, a tablet computer, asmartphone, a cellular telephone, a network appliance, a set-top box, apersonal digital assistant (PDA), a toy, a controller, a digital signalprocessor, a game console, a device controller, a computational enginewithin an appliance, a consumer-electronic device, a portable computingdevice or a portable electronic device, a personal organizer, and/oranother electronic device.

Moreover, optical source 802 can be used in a wide variety ofapplications, such as: communications (for example, in a transceiver, anoptical interconnect or an optical link, such as for intra-chip orinter-chip communication), a radio-frequency filter, a bio-sensor, datastorage (such as an optical-storage device or system), medicine (such asa diagnostic technique or surgery), a barcode scanner, metrology (suchas precision measurements of distance), manufacturing (cutting orwelding), a lithographic process, data storage (such as anoptical-storage device or system) and/or entertainment (a laser lightshow).

The foregoing descriptions of embodiments have been presented forpurposes of illustration and description only. They are not intended tobe exhaustive or to limit the present description to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present description. The scopeof the present description is defined by the appended claims.

What is claimed is:
 1. An integrated laser, comprising: a firstreflective silicon optical amplifier (RSOA); a second RSOA; an opticalring resonator; a symmetric power splitter comprising a reciprocalnetwork having a first port, a second port, a third port and a fourthport, wherein light entering the first port is approximately equallysplit with a 90-degree phase difference between the third port and thefourth port, and wherein light entering the second port is approximatelyequally split with a 90-degree phase difference between the third portand the fourth port; a first optical waveguide coupled between the firstport and the first RSOA; a second optical waveguide coupled between thesecond port and the second RSOA; a third optical waveguide coupled tothe third port, which channels light emanating from the third port inproximity to the optical ring resonator to cause optically coupled lightto circulate in a first direction in the optical ring resonator; afourth optical waveguide coupled to the fourth port, which channelslight emanating from the fourth port in proximity to the optical ringresonator to cause optically coupled light to circulate in a seconddirection, which is opposite to the first direction, in the optical ringresonator; and an output optical waveguide, which is optically coupledto the optical ring resonator.
 2. The integrated laser of claim 1,wherein light emanating from the first RSOA is split by the symmetricpower splitter between the third and fourth optical waveguides, whichcauses optically coupled light to circulate in opposite directions inthe optical ring resonator, thereby causing optically coupled light toreflect back through the third and fourth optical waveguides to thesymmetric power splitter, wherein the reflected light is combined anddirected back to the first RSOA, wherein no reflected light is directedback to the second RSOA due to phase cancellation, and wherein the firstRSOA, the first, third and fourth optical waveguides, the symmetricpower splitter and the optical ring resonator collectively form a firstlasing cavity for the integrated laser; and wherein light emanating fromthe second RSOA is split by the symmetric power splitter between thethird and fourth optical waveguides, which causes optically coupledlight to circulate in opposite directions in the optical ring resonator,thereby causing optically coupled light to reflect back through thethird and fourth optical waveguides to the symmetric power splitter,wherein the reflected light is combined and directed back to the secondRSOA, wherein no reflected light is directed back to the first RSOA dueto phase cancellation, and wherein the second RSOA, the second, thirdand fourth optical waveguides, the symmetric power splitter and theoptical ring resonator collectively form a second lasing cavity for theintegrated laser.
 3. The integrated laser of claim 1, wherein only oneof the first RSOA and the second RSOA is a currently active RSOA that ispowered on, and wherein the other RSOA is maintained as a spare RSOAthat is not powered on; and wherein the integrated laser furthercomprises, a failure-detection mechanism that detects a failure ordegradation of the first and/or the second RSOA, and a switchingmechanism that powers off the currently active RSOA and powers on thespare RSOA when the failure-detection mechanism detects that thecurrently active RSOA has failed.
 4. The integrated laser of claim 1,wherein a gain bandwidth of the first RSOA and a gain bandwidth of thesecond RSOA have different overlapping frequency ranges; and whereinonly one of the first RSOA and the second RSOA is powered on at a giventime depending on a desired wavelength to facilitate tuning theintegrated laser over a combined frequency range that includes theoverlapping frequency ranges of the first and second RSOAs.
 5. Theintegrated laser of claim 1, wherein a gain bandwidth of the first RSOAand a gain bandwidth of the second RSOA have different distinctfrequency ranges; and wherein the first RSOA and the second RSOA areboth powered on at the same time, whereby the integrated lasersimultaneously outputs laser emissions in two different wavelengthbands.
 6. The integrated laser of claim 1, wherein the first RSOA andthe second RSOA are located on one or more RSOA semiconductor chips,wherein the RSOA semiconductor chips are separate from a semiconductorchip that includes the symmetric power splitter, the first, second,third and fourth optical waveguides, the optical ring resonator and theoutput waveguide.
 7. The integrated laser of claim 1, wherein theoptical ring resonator comprises two optical ring resonators, which areoptically coupled in a Vernier configuration that facilitates tuning theoptical ring resonator over a wider spectral range than a single opticalring resonator can be tuned over.
 8. The integrated laser of claim 1,wherein the output optical waveguide has two ends that function as twooutputs of the integrated laser.
 9. The integrated laser of claim 8,wherein the two ends of the output optical waveguide feed into two armsof a Mach-Zehnder modulator (MZM), which modulates signals received fromthe output optical waveguide.
 10. The integrated laser of claim 8,further comprising one or more additional output optical waveguides,which are optically coupled to the optical ring resonator, and whereineach additional output waveguide has two ends that function as outputsof the integrated laser.
 11. The optical source of claim 8, furthercomprising a modulator coupled to each output of the integrated laser,wherein each output provides an optical carrier signal that is modulatedby the associated modulator to communicate information.
 12. Theintegrated laser of claim 1, wherein the symmetric power splittercomprises one of: a directional coupler; and a multi-modeinterferometer.
 13. A system, comprising: at least one processor; atleast one memory coupled to the at least one processor; and anintegrated laser for communicating optical signals generated by thesystem, wherein the integrated laser includes: a first reflectivesilicon optical amplifier (RSOA); a second RSOA; an optical ringresonator; a symmetric power splitter comprising a reciprocal networkhaving a first port, a second port, a third port and a fourth port,wherein light entering the first port is approximately equally splitwith a 90-degree phase difference between the third port and the fourthport, and wherein light entering the second port is approximatelyequally split with a 90-degree phase difference between the third portand the fourth port; a first optical waveguide coupled between the firstport and the first RSOA; a second optical waveguide coupled between thesecond port and the second RSOA; a third optical waveguide coupled tothe third port, which channels light emanating from the third port inproximity to the optical ring resonator to cause optically coupled lightto circulate in a first direction in the optical ring resonator; afourth optical waveguide coupled to the fourth port, which channelslight emanating from the fourth port in proximity to the optical ringresonator to cause optically coupled light to circulate in a seconddirection, which is opposite to the first direction, in the optical ringresonator; and an output optical waveguide, which is optically coupledto the optical ring resonator.
 14. The system of claim 13, wherein lightemanating from the first RSOA is split by the symmetric power splitterbetween the third and fourth optical waveguides, which causes opticallycoupled light to circulate in opposite directions in the optical ringresonator, thereby causing optically coupled light to reflect backthrough the third and fourth optical waveguides to the symmetric powersplitter, wherein the reflected light is combined and directed back tothe first RSOA, wherein no reflected light is directed back to thesecond RSOA due to phase cancellation, and wherein the first RSOA, thefirst, third and fourth optical waveguides, the symmetric power splitterand the optical ring resonator collectively form a first lasing cavityfor the integrated laser; and wherein light emanating from the secondRSOA is split by the symmetric power splitter between the third andfourth optical waveguides, which causes optically coupled light tocirculate in opposite directions in the optical ring resonator, therebycausing optically coupled light to reflect back through the third andfourth optical waveguides to the symmetric power splitter, wherein thereflected light is combined and directed back to the second RSOA,wherein no reflected light is directed back to the first RSOA due tophase cancellation, and wherein the second RSOA, the second, third andfourth optical waveguides, the symmetric power splitter and the opticalring resonator collectively form a second lasing cavity for theintegrated laser.
 15. The system of claim 13, wherein only one of thefirst RSOA and the second RSOA is a currently active RSOA that ispowered on, and wherein the other RSOA is maintained as a spare RSOAthat is not powered on; and wherein the integrated laser furthercomprises, a failure-detection mechanism that detects a failure ordegradation of the first and/or the second RSOA, and a switchingmechanism that powers off the currently active RSOA and powers on thespare RSOA when the failure-detection mechanism detects that thecurrently active RSOA has failed.
 16. The system of claim 13, wherein again bandwidth of the first RSOA and a gain bandwidth of the second RSOAhave different overlapping frequency ranges; and wherein only one of thefirst RSOA and the second RSOA is powered on at a given time dependingon a desired wavelength to facilitate tuning the integrated laser over acombined frequency range that includes the overlapping frequency rangesof the first and second RSOAs.
 17. The system of claim 13, wherein again bandwidth of the first RSOA and a gain bandwidth of the second RSOAhave different distinct frequency ranges; and wherein the first RSOA andthe second RSOA are both powered on at the same time, whereby theintegrated laser simultaneously outputs laser emissions in two differentwavelength bands.
 18. The system of claim 13, wherein the output opticalwaveguide has two ends that function as two outputs of the integratedlaser.
 19. The system of claim 18, wherein the two ends of the outputoptical waveguide feed into two arms of a Mach-Zehnder modulator (MZM),which modulates signals received from the output optical waveguide. 20.A method for generating optical signals, comprising: generating anoptical signal by powering up a first RSOA while leaving a second RSOAunpowered; coupling the generated optical signal through a first opticalwaveguide to a first port of a symmetric power splitter, wherein thesymmetric power splitter comprises a reciprocal network having a firstport, a second port, a third port and a fourth port, wherein lightentering the first port from the first optical waveguide isapproximately equally split with a 90-degree phase difference betweenthe third port and the fourth port, and wherein light entering thesecond port from a second optical waveguide, which is coupled to thesecond RSOA is approximately equally split with a 90-degree phasedifference between the third port and the fourth port; channeling lightemanating from the third port through a third optical waveguide thatpasses in proximity to an optical ring resonator to cause opticallycoupled light to circulate in a first direction in the optical ringresonator; channeling light emanating from the fourth port through afourth optical waveguide that passes in proximity to the optical ringresonator to cause optically coupled light to circulate in a seconddirection, which is opposite to the first direction, in the optical ringresonator; channeling light that is optically coupled from the opticalring resonator and reflected back into the third and fourth opticalwaveguides through the symmetric power splitter, wherein the reflectedlight is combined and directed back to the first RSOA, wherein noreflected light is directed back to the second RSOA due to phasecancellation, and wherein the first RSOA, the first, third and fourthoptical waveguides, the symmetric power splitter and the optical ringresonator collectively form a first lasing cavity for the integratedlaser; and providing an output through an output optical waveguide,which is coupled to the optical ring resonator.
 21. An integrated laser,comprising: a first reflective silicon optical amplifier (RSOA); asecond RSOA; a symmetric mirror having a first input port, a secondinput port and one or more output ports; a symmetric power splittercomprising a reciprocal network having a first port, a second port, athird port and a fourth port, wherein light entering the first port isapproximately equally split with a 90-degree phase difference betweenthe third port and the fourth port, and wherein light entering thesecond port is approximately equally split with a 90-degree phasedifference between the third port and the fourth port; a first opticalwaveguide coupled between the first port of the symmetric power splitterand the first RSOA; a second optical waveguide coupled between thesecond port of the symmetric power splitter and the second RSOA; a thirdoptical waveguide that channels light emanating from the third port ofthe symmetric power splitter into the first port of the symmetricmirror, which returns reflected light back through the third opticalwaveguide into the third port of the symmetric power splitter; and afourth optical waveguide that channels light emanating from the fourthport of the symmetric power splitter into the second port of thesymmetric mirror, which returns reflected light back through the fourthoptical waveguide into the fourth port of the symmetric power splitter;wherein the one or more output ports of the symmetric mirror provide oneor more outputs for the ring laser.